1. Field of the Invention
The invention relates to a method for polishing a semiconductor substrate and an apparatus for the same, and more particularly to a method for chemically and mechanically polishing a semiconductor substrate and an apparatus for the same.
2. Description of the Related Art
In order to fabricate a semiconductor integrated circuit including a multi-interconnection layer in which a plurality of interconnection layers are arranged in three dimensions, it is necessary to flatten surfaces of interlayer insulative layers (silicon oxide layers) disposed between multi-interconnection layers. Namely, when a silicon oxide layer is deposited by CVD subsequent to the formation of aluminum interconnection layers on a first layer or a lowermost layer, the aluminum interconnection layers cause irregularities on a surface of the deposited silicon oxide layer. When second aluminum interconnection layers are to be patterned on the silicon oxide layer having such irregularities in a photolithography step and a dry etching step, the irregularities pose such problems that focus for exposure of resist patterning is not able to be adjusted and that residue of dry etching remains at steps of the irregularities. Thus, Japanese Patent Publication No. 5-30052 has suggested that such irregularities present on a surface of an interlayer insulative layer are removed by polishing to thereby flatten the surface. Specifically, the Publication has suggested the method including the steps of supplying process liquid to a polishing cloth lying on a rotatable surface table, and compressing a silicon substrate onto the polishing cloth to thereby remove the irregularities present on an interlayer insulative layer. The polishing of a silicon oxide layer is accomplished by both a chemical etching function of silicon oxide and a mechanical function caused by friction between the irregularities and abrasive particles. Japanese Unexamined Patent Public Disclosure No. 4-75338 has suggested the use of process liquid comprising ammonia aqueous solution containing, as abrasive particles, silica particles having a diameter of approximately 40 nm in the range of 10 wt % to 30 wt %.
In polishing to remove micron order irregularities present on an interlayer insulative layer, a structure of a substrate holder in a polishing apparatus is quite important for carrying out uniform polishing within a substrate. For instance, if irregularities are present on a surface of such a substrate holder, uniformity of polishing a substrate would be degraded, even if such irregularities are minute. Alternatively, if particles exist between a substrate holder and a reverse surface of a substrate, such particles cause polishing speed of an interlayer insulative layer to locally change, even if such particles are of micron order. In addition, a substrate may be damaged at a reverse surface thereof by contact with a substrate holder. Thus, how a substrate is held exerts a great influence on the flattening of a surface of a thin layer, which constitutes a semiconductor, including a polysilicon layer and a metal layer as well as an interlayer insulative layer
There have been the following four conventional methods A to D for holding a substrate.
A. In method A, as illustrated in FIG. 1A, a substrate 1 is fixedly adhered to a substrate holder 13 through an adhesive 14 such as hot melt wax. The substrate 1 adhered to the substrate holder 13 is compressed onto a polishing cloth 9 lying on a surface table 10, and then a rotation shaft 8 rotates the substrate holder 13 and hence the substrate 1 on the polishing cloth 9. Thus, a surface of the substrate 1 is polished to thereby remove irregularities present on the surface of the substrate 1. For instance, such a method A has been suggested in Japanese Unexamined Patent Public Disclosure No. 5-154760 laid open on Jun. 22, 1993, and Japanese Unexamined Patent Public Disclosure No. 6-224165, laid open on Aug. 12, 1994, which is based on U.S. patent application Ser. No. 07/961565 filed on Oct. 15, 1992 and assigned to Applied Materials Incorporated.
B. In method B, as illustrated in FIG. 1B, the substrate 1 is sucked to the substrate holder 13 by a vacuum pump 17 through a hole 3 formed in the substrate holder 13. Thus, the substrate 1 is polished on the polishing cloth 9 with the substrate 1 being vacuum-sucked to the substrate holder 13. For instance, such a method B has been suggested in Japanese Unexamined Patent Public Disclosure No. 5-277908, laid open on Oct. 26, 1993, which is based on U.S. patent application Ser. No. 07/826,394 filed on Jan. 27, 1992 and assigned to Micron Technology Incorporated, and also in the above mentioned Japanese Unexamined Patent Public Disclosure No. 6-224165.
C. In method C, as illustrated in FIG. 1C, a pad 15 which contains water is attached to the substrate holder 13 at its bottom surface, and a guide ring 4' is fixedly secured to the substrate holder 13 at its periphery. The substrate 1 is compressed onto the polishing cloth 9 to thereby remove irregularities with being held in position by surface tension of water contained in the pad 15.
D. In method D, as illustrated in FIG. 1D, the substrate holder 13 is provided with the pad 15 and the guide ring 3. While the substrate 1 is held in position by surface tension of water contained in the pad 15, a pump 18 applies pressurized air 16 (backing pressure) to the substrate 1 though the hole 3 of the substrate holder 13 to thereby prevent deformation of the pad 15. The substrate 1 is compressed onto the polishing cloth 9 to thereby remove irregularities thereof in the same way as the above mentioned methods A to C.
However, the above-mentioned conventional methods have problems as follows.
First, the method A, in which the substrate 1 is fixed to the substrate holder 13 by the adhesive 14 as illustrated in FIG. 1A, additionally requires the steps of applying the adhesive 14 to a reverse surface of a substrate to be polished, and cleansing and removing the adhesive 14 from the substrate 1. Thus, the method A is not suitable to a semiconductor fabrication process which requires mass-productivity, for instance, in a step for flattening an interlayer insulative layer.
In the method B illustrated in FIG. 1B, abrasive particles may be attracted to the reverse surface of the substrate 1 and enter between the reverse surface of the substrate 1 and the substrate holder 13 while the substrate is being polished, and such abrasive particles deteriorate the flatness of a polished surface of the substrate 1. In addition, since the substrate 1 makes direct contact with the substrate holder head 13, the substrate 1 may be damaged at its reverse surface.
In the method C, in which the pad 15 and the guide ring 4' is secured to the substrate holder 13 as illustrated in FIG. 1C, the pad 15 may be deformed in long use with the result that the flatness of a polished substrate is degraded and hence uniformity of polishing is also degraded. Further, while polishing, the substrate 1 may exert a stress on the guide ring 4'. In general, the adhesive 14 does not have water-resistance, and hence the guide ring 4' tends to peel off from the substrate holder 13. If alkaline slurry is to be used as process liquid, the guide ring 4' is more likely to peel off.
In the method D, in which the substrate holder 13 and the pad 15 are formed with the through hole 3, as illustrated in FIG. 1D, through which the substrate 1 is vacuum-chucked while the substrate 1 is being transferred, and through which the air 16 is applied to the pad 15 to prevent deformation of the pad 15 while the substrate 1 is being polished, it is relatively easy to automate securing the substrate 1 to the substrate holder 13 and releasing the substrate 1 from the substrate holder 13. However, since the reverse surface of the substrate 1 is kept in contact with the air 16 while the substrate 1 is being polished, the reverse surface of the substrate 1 tends to become dry, and hence slurry of process liquid reaching the reverse surface of the substrate 1 is prone to become dry and stick to the reverse surface of the substrate 1. In addition, even if the deformation of the pad 15 is prevented by the air 16 while polishing, the pad 15 is worn out in long use. Thus, it is impossible to avoid local deformation of the pad 15.